If it were possible to give ions a mass-dependent velocity, and then start all the ions on their way at the same time from the same place with the same impetus, the ions would spread out along the flight path according to their velocity, the faster ones advancing over the slower ones. This simple concept is the basis of time-of-flight mass spectrometry where a detector is placed at the end of the flight path and the detector response is recorded as a function of the time elapsed since the ion packet's release from the ion source. Peaks in detector response occur as ions of increasing mass-to-charge ratio (m/z) arrive at the detector where the roman m indicates mass in atomic mass units. Interpretation of the arrival time in terms of the m/z value gives the relative abundance of ions of all m/z values in the original ion packet.
Time-of-flight (TOF) has been pursued as the basis for mass spectrometers since the 1940s. The usual method of ion acceleration in TOF instruments is constant energy. However, a TOF mass spectrometer using constant momentum acceleration was built by Wolff and Stevens in 1953 (Wolff, M. M.; Stephens, W. E. Rev. Sci. Instr. 1953, 24, 616-617). Limitations in achievable resolution and the speed of available detection electronics made TOF mass spectrometry uncompetitive with instruments based on the emerging quadrupole mass filters. Now, due to improvements in both the previously limiting elements, TOF mass spectrometry has become the method of choice for many applications. Its principal advantages over other methods of mass analysis are its ability to produce full spectra for each ion packet and to do so at a very high rate of spectral generation. Since the flight times in TOF-mass spectrometry are very short (on the order of 100 μs or less) the electronics for recording ion arrival times and the ion abundance at each time can be expensive and can also be the limiting factor in dynamic range and maximum ion detection rate.
Distance-of flight (DOF) is another possible approach to the application of m/z-dependent velocity for mass analysis. In this approach, one would release the ions from the source with m/z dependent velocities and then, at a specific time after release from the source, determine the number of ions at each unit of distance along the flight path. To use a chromatographic analogy, TOF-MS is to column chromatography what DOF-MS is to thin-layer chromatography. DOF-MS retains the advantages of TOF-MS mentioned above but substitutes an array of non-time-dependent detectors for a single high-speed detection system. Array detectors have the advantage of allowing signal accumulation within each element over many ion packet releases for improved dynamic range and signal-to-noise ratio. They also avoid the need for high-speed electronics as the m/z assignment is made by the element at which the ion is detected, not the time of its detection. For all applications requiring the high quantitative precision of Faraday cup detectors, array detection geometry allows multiple parallel detection.
The use of an array of detectors in which ions of different m/z values fall on different detectors requires a spatial m/z dispersive device. The most often used device is the Mattauch-Herzog mass spectrograph (Sinha, M. P., Wadsworth, M. Rev. Sci. Instr. 2005, 76, 1-8; Barnes, J. H. IV, Schilling, G. D., Sperline, R., Denton, M. B., Young, E. T., Barinaga, C. J., Koppenaal, D. W., and Hieftje, G. M. Anal. Chem. 2004, 76, 2531-2536; Schilling, G. D., Andrade, F. J., Barnes, J. H. IV, Sperline, R. P., Denton, M. B., Barinaga, C. J., Koppenaal, D. W., and Hieftje, G. M. Anal. Chem. 2006, 78, 4319-4325). Array detectors have not been previously employed in mass spectrometry in systems depending on the relative flight distance of ions with m/z-dependent velocities in the same packet. A totally different approach is found in U.S. patent application US2004/0206899 A1 by Webb. et al. in which ions of various m/z in a packet are given the same velocity and then sorted according to their different energies onto an array detector.
Time-of-flight mass spectrometers are not as simple as the above discussion implies, due largely to the fact that all the ions of the same m/z value do not start from the same place or with the same velocity and thus do not arrive at the detector at the same time. Methods to compensate for this dispersion of initial space and velocity have been the object of a great deal of study since TOF-MS was first developed. The same problems beset DOF-MS, but the same solutions are not suitable.